1. Field of the Invention
The present invention relates to an optical disk having concavities such as pits and grooves and a manufacturing method of an original optical disk for producing such optical disks.
2. Description of Related Art
An optical disk is a recording medium in which information is recorded by pits and phase-change marks which can be read by irradiating a light beam. Such optical disk is drawing much attention as a storage medium having a high density and large capacity memory for use in various information processing systems, including personal computers, and as a recording medium for recording audio and video signals. Presently, research and development to further increase the density and capacity of the optical disks are being actively conducted.
Generally, optical disks are manufactured by creating a stamper from an original disk via a plating step; the steps of duplicating a large number of disk substrates from the stamper by injection molding; and forming a reflection film (such as an aluminum film) or a recording film (such as a phase-change film or a magneto-optic film) on the disk substrate. The original disk is normally created by the steps of: applying a photoresist film on a glass substrate; irradiating an exposing light beam on the photoresist film while being focused by an objective lens; and developing it. Here, pits corresponding to information are formed on the original disk by modulating the intensity of the exposing light beam corresponding to signals of the information to be recorded, and grooves are created on the original disk by irradiating the exposing light beam whose intensity is uniform.
The optical disk, the replica, on which the information is recorded, in the form of pits, may be obtained by creating the stamper from the original disk having the aforementioned pits and by injection molding the reflection film on the disk substrate after transferring the pits on the disk substrate, which made of resin, by using the stamper.
Meanwhile, the replica optical disk having grooves on the recording film for tracking-guiding the light beam may be obtained by creating the grooves on the original disk and by creating the recording film on the disk substrate. In this case, information may be recorded as marks such as phase-change marks on the recording film along the grooves.
The information may be reproduced from the optical disk in which such information is recorded by obtaining reproduced signals by irradiating a reproducing light beam to the optical disk and by detecting reflected lights from the pits and marks by a photodetector.
To increase the density of the optical disk in order to respond to the demand of increased capacity of the optical disk, it is essential that, pits on the optical disk be micronized and the width of a groove be minimized to reduce the track pitch. As for the micronization of pits and groove width in the original optical disk, a micro-processing technology of forming pits and grooves of about 0.2 .mu.m in width is now being developed by shortening a wavelength of the exposing light beam or by using a technology of exposing using an electron beam.
A micro-pit creating technology using a dry-etching technology is also being developed because very small pitches and marks can be read while suppressing cross talk between the adjacent tracks by sharpening inclination of wall faces of pits and grooves.
FIGS. 6 and 7 show sectional profiles of a pit of an optical disk, in the track width direction, created by an original optical disk produced according to prior art methods. The pit in FIG. 6 has side walls along the track direction which are relatively mildly inclined, i.e., which are normally tapered (having an inclination by which the closer the bottom side, the narrower the width of an opening becomes). Meanwhile, the pit in FIG. 7 is normally tapered while having sidewalls along the track direction sharply inclined. The taper having such sharp inclination, as in FIG. 7, may be achieved by using dry-etching in developing the photoresist film.
However, when the pits are micronized in order to densify the optical disk, it becomes difficult to mold the optical disk substrate because the fine pits on the original optical disk must be transferred to the optical disk substrate, i.e., a replica, via the stamper. In particular, it causes problems in mass-producing the replicas by injection molding since resin cannot fully enter within the pits when the inclination of the pit side wall is as sharp as shown in FIG. 7. Thus, not only is the transferability deteriorated but the releaseability of the optical disk substrate from the stamper after the transfer is also adversely affected. These problems apply also to the case of the optical disk whose groove width is reduced.
Furthermore, when a multi-layered recording film is to be formed on this optical disk substrate according to the above-mentioned methods, it becomes difficult for the recording film material to enter within the pits or the grooves to form the recording film when the width of the pit and the groove is narrow and/or the inclination of the side wall is sharp.
As described above, the prior art technology has had the problems of the deterioration of the transferability and the releaseability of the optical disk substrate with respect to the stamper when: pits and grooves are micronized in order to densify the optical disk; the inclination of the side wall of the pit and the groove is sharpened along the side wall; and forming a recording film when the optical disk is a rewritable optical disk; the recording film is a multi-layered film in particular.